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Ipomoviruses are distinguished from members of other genera by their mode of transmission being by whiteflies, and by their separate branching on phylogenetic analyses.
Virions are flexuous filaments 800–950 nm long.
Virion Sedimentation coefficient S20,w is 155S for sweet potato mild mottle virus (SPMMV).
Virions contain a single molecule of linear, positive-sense ssRNA of about 9.7 kb; virions contain 5% RNA by weight.
The viral coat (capsid) protein (CP) is a single polypeptide of 302–378 aa (35–41 kDa).
Ipomoviruses exhibit unusual structural variability. The structure and organization of the SPMMV genome is similar to that of members of the genus Potyvirus (Figure 2.Potyviridae), but some motifs of HC-Pro and CP characteristic of members of the genus Potyvirus are incomplete or missing, which may account for its vector relations. The unusually large P1 protein (83 kDa) of SPMMV contains no obvious AlkB domain and hence differs from that found in members of the genus Brambyvirus. Cucumber vein yellowing virus (CVYV) and squash vein yellowing virus (SqVYV) differ from SPMMV by encoding two P1-like serine proteases (P1a and P1b) but no HC-Pro. P1b functions as a suppressor of RNA silencing (Li et al., 2008). Cassava brown streak virus (CBSV) differs from SPMMV by having no HC-Pro and differs from CVYV and SqVYV by having only P1b which suppresses silencing (Figure 1.Ipomovirus). Additionally, cassava brown streak virus (CBSV) contains a Maf/HAM1-like sequence recombined into the NIb/CP junction, which can accommodate heterologous genes in engineered infectious potyvirus clones. Homology of HAM1h with cellular Maf/HAM1 NTP pyrophosphatases suggests that HAMh1 might intercept non-canonical NTPs to reduce mutation rates of viral RNA.
The natural host range of SPMMV is wide, with more than nine families susceptible, whereas the host range of CBSV, CVYV and SqVYV is less known apart from the hosts which they have been found to infect in the field (Winter et al., 2010).
CBSV, CVYV and SqVYV are transmitted by the whitefly Bemisia tabaci in a non-persistent manner. B. tabaci may also be the vector of SPMMV, but this is not fully confirmed. All ipomoviruses are transmissible experimentally by mechanical inoculation and by grafting.
Moderately immunogenic. No serological relationships with other members of the family Potyviridae have been found.
Ipomovirus: from Ipomea and mosaic, the symptoms induced on the host of members of the type species Sweet potato mild mottle virus.
See discussion under family description.
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